Accelerometers are used in various applications, such as measuring the magnitude of an earthquake and gathering seismic data, detecting the magnitude of a collision during a car accident, and detecting the tilting direction and angle of a mobile phone or a game console. As micro-electro-mechanical systems (MEMS) technology continues to progress, nano-scale accelerometers have been widely used in commercial applications.
There are two kinds of accelerometers which are commonly used: piezoresistive accelerometers and capacitive accelerometers. A piezoresistive accelerometer, such as described in Chinese Patent Application No. 200480003916.7, usually comprises a mass and beams, with piezo elements provided on the beam. The mass moves according to the acceleration and causes the beam to deform, which also causes a change of resistance. However, under situations when there is no acceleration or the acceleration magnitude is relatively small, the beam will not deform significantly, and there is no significant change in resistance. The accelerometer can only detect acceleration when the magnitude of acceleration is large enough to cause the deformation of the beam. Therefore, the piezoresistive accelerometer has low accuracy.
A capacitive accelerometer, such as described in U.S. Pat. No. 6,805,008, also includes beams and a mass. When acceleration is present, the frame of the accelerometer will move according to the acceleration direction, but due to inertia, there is little displacement for the mass. Thus, the change in gap distance between the mass and another electrode causes a change in capacitance.
Both kinds of accelerometers are manufactured by micro fabrication techniques and have the characteristics of small size and low manufacturing cost. However, the beam is an elastic beam, and there are only four beams connecting the mass with the frame. Thus, when the outer frame moves, the displacement of each beam is relatively large. Also, each beam will not create a uniform displacement and deformation, which leads to asymmetrical vibration modes. Also, this type of accelerometer only measures the acceleration by measuring the change in capacitance in the vertical direction. When there is acceleration in the vertical direction, such as a falling object, the measurement accuracy of this type of accelerometer is relatively high; but when there is only acceleration in the horizontal direction, such as side-to-side shaking, the measurement accuracy of this type of accelerometer is relatively low.
Furthermore, both types of accelerometers can only measure the acceleration in one direction. In order to measure the acceleration in three directions, three separate accelerometers are required, which not only increases the cost, but requires high installation accuracy.